The present invention relates to ram air turbines, and more particularly, exemplary embodiments of the present invention are directed to speed control of fixed pitch ram air turbines.
A Ram Air Turbine (RAT) is commonly an aircraft power system which extracts energy from the air stream of an aircraft in order to provide emergency or auxiliary electrical and/or hydraulic power. A RAT conventionally includes a turbine assembly, strut assembly, generator and/or hydraulic pump, and actuator. During operation, the turbine assembly, which is attached to the strut, is deployed into the air stream by means of the actuator. Once deployed, the turbine is driven by the energy of the air stream surrounding the aircraft. The rotational motion of the turbine is then converted into electrical and/or hydraulic power.
RAT turbine speed is typically controlled by a closed loop mechanical governing mechanism which uses counterweights to adjust the pitch of the turbine blades. Governing (speed control) is needed to control RAT system power quality and limit structural loads in the RAT rotating components. If the RAT turbine is not governed, over-speed conditions can occur which generate large, unwanted forces, and disrupt or damage generators of RATs.
Another form of RAT, dubbed “ducted” or “inboard,” includes an enclosed turbine which is integral to a main aircraft fuselage or attached to the exterior of the aircraft via the use of a pod or similar device. Inboard RATs are driven by an axial-style, fixed pitch turbine which utilizes a shroud or ducting for channeling the input high velocity air. Inboard RATs can be used for both supplemental and emergency power generation applications.
Conventional inboard RAT designs use fixed pitched turbines and actuating doors or throttle-like control mechanisms in order to limit an amount of air flow entering or exiting the turbine. However, such designs have an inherent response lag and cannot account for short duration transient load drops that allow the turbine to accelerate and can induce over-speed conditions.